1. Field of the Invention
This invention relates to apparatus and method for the manufacture of a continuous sheet of flat glass by supporting molten glass on a pool of molten metal while forming and cooling the glass. More particularly, this invention relates to apparatus and method for the delivering of the molten glass onto a pool of metal for formation.
2. Description of the Prior Art
Molten glass may be delivered onto molten metal and formed into a continuous sheet or ribbon of glass. In the method according to the patents of Pilkington, U.S. Pat. No. 3,083,551 and U.S. Pat. No. 3,220,816, the molten glass is delivered through a narrow channel and over a lip from which the molten glass falls onto the molten metal and spreads outwardly on the molten metal. The method of Pilkington utilizing a free-fall of the glass presents problems in control of the molten glass sheet as it leaves the pool it has formed by falling onto the tin, and further, presents problems in forming glass sheet that is not near the equilibrium thickness of molten glass floating on tin.
In order to overcome some of the disadvantages of the process when the glass is allowed to freely fall, a process was developed wherein a wide molten ribbon of glass was delivered onto the tin bath and then attenuated with very little change in width between the width of channel or canal from the furnace and the width of the ribbon or sheet of glass on the molten tin. Such a process is disclosed in U.S. Pat. No. 3,843,346 to Edge et al. This process allows good delivery control as the glass is smoothly delivered to the forming chamber. However, while delivery to the forming chamber is smooth the glass sheet is likely to have a thickness contour causing areas of visual distortion closely following the temperature and viscosity profile the molten glass has as it is delivered to the forming chamber's bath of molten metal.
In both the free fall delivery channel and the wide forming entrance there is a tendency within the forming entrance for the molten glass to become cooler at the edges by heat loss in those areas and also the flow rate is slowed by frictional drag at the edges. Therefore, there is a tendency for a parabolic temperature and viscosity distribution of high velocity and low viscosity hotter glass at the center of the canal or delivery channel and cooler, slower moving glass at the edges of the delivery channel. This creates thickness contour control difficulty as the higher temperature areas will stretch to greater degree during the attenuation or attenuation and stretching in the forming chamber. The cooler edges also may devitrify and lodge in the forming entrance. The devitrified glass lodged in the forming entrance may break off in pieces which disrupt the forming chamber operation.
In order to try to even the flow of the glass and control the parabolic viscosity temperature distribution of the glass, it has been proposed that the tweel be shaped to restrict flow of the molten glass at the center portion of the delivery channel. Such shaped tweel members are disclosed in U.S. Pat. No. 3,973,940 and U.S. Pat. No. 3,442,636 wherein a tweel having a lower center portion than edges is disclosed. However, such tweels, as they only contact one point of the glass stream entering the forming chamber, create a localized flow pattern around the tweel but do not significantly change the basic temperature and viscosity characteristics of the glass within the entering channel.
Therefore, there remains a need for an improved system of glass delivery to a forming chamber such that the temperature and viscosity distribution would be improved. At present in order to alleviate the parabolic temperature and viscosity distribution in the molten sheet glass, the use of overhead cooling in the forming chamber and in the forming entrance upstream of the tweel to cool the center portions of the glass has been practiced. However, these methods are difficult to control so as to give even cooling and not introduce additional areas of uneven temperature causing distortion at the top and bottom surfaces of the glass sheet. It would be desirable if the forming chamber could be successfully operated without substantial overhead cooling of the glass sheet.
As set forth above there remains a need for a method of glass delivery to the forming chamber that will aid in formation of a glass with an even thickness contour. Present systems deliver a hot center and relatively cold edges that stretch non-uniformly as they do not flow uniformly resulting in an uneven surface contour. There remains a need for an entrance to the forming chamber that will not permit devitrification of glass or accumulation of devitrified glass at the sides of the entrance during normal operation. The above difficulties are further exacerbated by increases in tonnage put through the forming system and furnace conditioners that do not have return flow. Also, conditioners which provide no return flow may deliver glass to the forming entrance that already has some temperature and viscosity profile.